Crude oil produced from subsurface reservoirs is a mixture of a number of different hydrocarbons; the most commonly found molecules are alkanes, cycloalkanes, aromatic hydrocarbons, or more complicated chemicals like asphaltenes. Each type of crude oil has a unique mix of molecules influencing the physical and chemical properties, like density and viscosity.
The alkanes, also known as paraffins, are saturated hydrocarbons with straight or branched chains which contain only carbon and hydrogen. Alkanes generally contain up to 50 carbon atoms. The cycloalkanes, also known as naphthenes, are saturated hydrocarbons which have one or more carbon rings. Cycloalkanes have similar properties to alkanes but have higher boiling points.
The aromatic hydrocarbons are unsaturated hydrocarbons which have one or more planar six-carbon rings called benzene rings containing six delocalized π-electrons. Aromatic compounds in crude oil are undesirable because of soot production during combustion and tend to burn with a sooty flame.
Moreover, sulphur is often incorporated in large aromatic molecules. Sulphur lowers the quality, and hence economic value, of crude oil because it leads to environmental problems and requires major refining to meet sulphur standards imposed on fuels in consuming countries.
The petroleum industry generally classifies crude oil by the geographic location it is produced in, its API gravity and its sulphur content. Crude oil is considered light if it has low density or heavy if it has high density, and it is referred to as sweet if it contains relatively little sulphur or sour if it contains higher amounts of sulphur.
Light crude oil is more desirable than heavy oil since it produces a higher yield of petrol, while sweet oil commands a higher price than sour oil because it has fewer environmental problems and requires less refining to meet sulphur standards imposed on fuels in consuming countries.
Aromatic molecules can exist in complex structures containing many rings and the non-bonding (called π-π stacking) interaction between these rings is strong and so pure compounds of even a few rings are solid at room temperature where the equivalent (in terms of number of carbon atoms) normal paraffin is a viscous liquid under the same conditions. Thus the greater the amount of aromatics in crude oil, the higher crude oil viscosity which is undesirable for the transport of the oil.
It is the ratio of these compound types (aromatics, paraffins and naphthenes) that impacts the quality, density, sourness and viscosity of the crude.
Moreover, from a health perspective, aromatic compounds are often carcinogenic so clearly aromatic compounds are also in this respect undesirable in crude oil.
In summary, aromatic compounds in crude oil    a) lower the quality of crude oil    b) lead to increased viscosity, lower mobility during oil transport    c) lead to sooty smoke and hence air pollution with negative impact on the environment    d) are carcinogenic and cause health problems and potential onset of cancers when in contact with human skin, lungs (when inhaled in the gas phase) etc.    e) often contain incorporated sulphur atoms in polycyclic aromatic hydrocarbons molecules    f) lead to higher refinery costs to remove the aromatics and polycyclic aromatics hydrocarbons (containing e.g. sulphur) from the crude oil
These molecules can be removed from the crude oil in upstream refining processes typically involving boiling point distillation. Aromatic hydrocarbons, heteroatomic aromatic compounds and polynuclear aromatic compounds are typically removed by phase transitive processes.
US 20090050315 discloses the reduction of the viscosity of crude oil through reductive biotransformation of aromatic components of the crude oil by enhancing the biochemical capabilities of the natural anaerobic microbial populations of the well. The method comprises providing injection water containing anaerobic microbial populations capable of reducing aromatic hydrocarbons.
Processes have been disclosed employing solid adsorbent materials for use in treating hydrocarbon feed streams to remove undesired compounds, including nitrogen and sulfur-containing compounds. For example, U.S. Pat. No. 4,846,962 discloses a process for selectively removing basic nitrogen compounds from solvent extracted oils by their absorption to a solid acidic polar-absorbent material. Following the solvent extraction process, the basic nitrogen compounds present with the desired oil fraction are contacted with adsorbents of the silica-alumina type, Ketjen high-alumina base (amorphous) and H-Y zeolite (crystalline) identified as being preferred. In addition, various treatments were applied to the adsorbents to improve their effectiveness. It was also disclosed that the adsorbents could be regenerated, e.g., by purging with a hot hydrogen gas stream.
US 20100147647 and US20090120842 disclose a treatment of crude oil feed stream to reduce the content of heteroatomic and polynuclear aromatic compounds containing nitrogen and sulfur by contacting the feed stream with one or more solid absorbent materials selected from attapulgus clay, alumina, silica gel and activated carbon in a mixing vessel to optimize the absorption of the undesired compounds, followed by subjecting the mixture to atmospheric flash distillation and vacuum flash distillation to recover presorbed boiling ranges of products having a lowered content of the undesired compounds.
U.S. Pat. No. 8,021,540 describes a method of desulfurizing a hydrocarbon oil, such as kerosene or gas oil by which sulfur compounds are diminished. The method comprises bringing a hydrocarbon oil containing thiophene compounds, benzothiophene compounds, dibenzothiophene compounds or aromatic hydrocarbons into contact with a solid acid catalyst and/or an activated carbon having a transition metal oxide supported thereon to thereby desulfurize the oil. The solid acid catalyst preferably is a solid ultrastrong-acid catalyst selected among sulfuric acid radical/zirconia, sulfuric acid radical/alumina, sulfuric acid radical/tin oxide, sulfuric acid radical/iron oxide, tungstic acid/zirconia, and tungstic acid/tin oxide.
There is general consensus that the average crude oil quality is trending down. The average API gravity and sulfur content of aggregate refinery crude slates varies by region; some regions process lighter, sweeter crude slates than others. However, over time, the average quality of the global crude slate has been declining gradually. Average API gravity has been decreasing. Average sulfur content has been increasing more rapidly, a trend likely to continue for the foreseeable future. Thus, aromatic compounds are highly undesirable constituents of crude oil. There is therefore a need in the industry for methods to remove these molecules from the crude oil in upstream refining processes.